1. Field of the Invention
The present invention relates to a method and device for controlling a common rail pressure in a common-rail type fuel injection system.
2. Description of the Prior Art
As for the fuel injection control in engines, a common-rail type fuel injection system has been known which provides a high injection pressure and performs optimum control on injection characteristics, such as fuel injection timing and the amount of fuel injected, according to the operating condition of the engine. The common rail type fuel injection system is a fuel injection system that stores in the common rail a fuel pressurized to a predetermined pressure by a pump and then injects the stored fuel from injectors into corresponding combustion chambers. To ensure that the pressurized fuel will be injected from each injector under optimum injection conditions according to the engine operating conditions, a controller controls the fuel pressure in the common rail and the operation of control valves for the injectors according to the operating conditions of the engine.
The conventional common-rail type fuel injection system will be described by referring to FIG. 5. The fuel is supplied to individual injectors 1 from a common rail 2 through branch pipes 3 that form a part of the fuel passage. The fuel, which was pumped by a feed pump 6 from a fuel tank 4 through a filter 5 and pressurized to a predetermined pressure, is delivered to a fuel pump 8 through a fuel pipe 7. The fuel pump 8 may, for example, be a so-called plunger type fuel supply pump driven by the engine which raises the fuel pressure to a high pressure determined by the operating condition of the engine and delivers the pressurized fuel through a fuel pipe 9 to the common rail 2. The fuel is then stored temporarily in the common rail 2 at the elevated pressure, from which it is supplied to individual injectors 1.
Normally there are provided two or more injectors 1 corresponding in number to cylinders in the engine (or according to the type of engine). These injectors 1 are controlled by a controller 12 to inject fuel supplied from the common rail 2 into the corresponding combustion chambers in optimum amounts and at optimum timings. Because the pressure at which the fuel is injected from the injectors 1 is equal to the pressure of the fuel stored in the common rail 2, the injection pressure is controlled by controlling the fuel pressure in the common rail 2.
The fuel released from the fuel pump 8 is returned to the fuel tank 4 through a return pipe 10. Of the fuel supplied from the branch pipes 3 to the injectors 1, the fuel that was not used for injection into the combustion chambers is returned to the fuel tank 4 through a return pipe 11. The controller 12 as an electronic control unit is supplied with signals from various sensors for detecting the engine operating condition, which include: engine cylinder determination and crank angle sensors for detecting an engine revolution speed Ne; an accelerator opening sensor for detecting the amount of accelerator operation Acc; a water temperature sensor for detecting the cooling water temperature; and an intake pipe inner pressure sensor for detecting the inner pressure of the intake pipe.
The controller 12, based on these signals, controls the fuel injection characteristics of the injectors 1, i.e., the fuel injection timing and the amount of fuel to be injected so that the engine output will become optimum for the current engine condition. The common rail 2 is provided with a pressure sensor 13 which detects the fuel pressure in the common rail 2 and sends the detection signal to the controller 12. The controller 12 controls the delivery pressure of the fuel pump 8 so that the fuel pressure in the common rail 2 remains constant even when the fuel in the common rail 2 is consumed for the injection from the injectors 1.
As disclosed in Japanese Patent Publication No. 60020/1985, the common-rail type fuel injection device controls the injection pressure to a target value according to the operating condition and at the same time calculates injection characteristics corresponding to the operating condition, i.e., the amount of fuel to be injected (determined by the fuel injection pressure and the fuel injection timing), and, according to the calculated results, controls the valve openings of the injectors for injecting high-pressure fuel, thereby realizing the fuel injection characteristics conforming to the engine operating state. The fuel pressure is raised by a pump and regulated by a pressure regulating valve to a predetermined injection pressure.
In the common-rail type fuel injection device, the common rail target pressure under normal (steady state) operating condition is so set that the exhaust emission performance is optimum. Under a transient state where the common rail target pressure is not constant, however, the exhaust emission performance may deteriorate unless an appropriate pressure control is provided. That is, in the common-rail type fuel injection device, although raising the fuel injection pressure can easily be done by raising the common rail pressure by operating the control valve of the fuel pump, the common rail pressure cannot be lowered except by fuel injection from the injectors or by leaking fuel from the injection system such as common rail.
Hence, in a situation where the accelerator pedal is quickly released as when a vehicle sharply decelerates from high speed, or where during a high-speed traveling the accelerator pedal is released to such an extent that the fuel is not injected from the injectors, the engine output revolution decreases and the common rail target pressure determined by the controller according to the result of detection of the amount of the accelerator pedal depression quickly becomes small. However, the actual common rail pressure cannot follow the rapid reduction in the target pressure and remains high. That is, because the fuel injection from the injectors are stopped, the fuel in the common rail is not consumed. Further, the fuel cannot be expected to leak within a short period of time from an injection system ranging from the common rail to the injectors. Hence, the actual common rail pressure continues to be high. In such a condition, when the fuel injection from the injectors is resumed, the fuel is injected at high pressure, increasing NOx and degrading the exhaust emission performance.
To deal with the lack of quick response on the part of the common rail pressure to the reduction in the common rail target pressure described above, Japanese Patent Laid-Open No. 117147/1988 discloses a fuel injection device incorporating a rapid pressure reducing system in the common rail. The rapid pressure reducing system is a fuel injection control device for internal combustion engines that delivers fuel by a variable displacement pump from the fuel tank to a fuel accumulator, supplies fuel from the fuel accumulator to the fuel injection valves, and controls the amount of fuel delivered by the variable displacement fuel pump so that the fuel pressure in the fuel accumulator detected by a pressure sensor matches the target fuel pressure determined according to the engine operating condition. When the fuel pressure of the fuel accumulator detected by the pressure sensor is higher than the target fuel pressure by a predetermined value or larger, a pressure reduction relief valve (solenoid valve) provided in the fuel accumulator is opened to release the high pressure fuel from the fuel accumulator through a relief pipe. With this fuel injection control device for internal combustion engines, when the vehicle is rapidly decelerated, the fuel pressure swiftly changes from the high-pressure state to the low-pressure state, preventing a delay in reducing the pressure of the fuel accumulator which would otherwise be caused by small fuel consumption at deceleration. The rapid pressure reduction system in the common rail, however, requires a relief valve and a relief pipe for pressure reduction, increasing the number of parts and therefore the cost of the fuel injection control device.